This invention relates to a method of remotely detecting the presence of rain by obtaining information from high frequency radio waves scattered from the sea surface. Such waves may have been generated by a skywave radar facility or they may be transmissions of opportunity radiated by HF broadcasters or other sources, man-made or natural.
Rainfall over the ocean has hitherto only been measured by ships at sea and, recently, by satellite-borne sensors. The former method is only able to collect data regularly on commercial shipping routes and irregularly at other parts of the globe, while the satellite techniques, which are still highly experimental, are limited to the narrow swathe below the satellite orbit.
By observing rainfall over the oceans a better understanding of the dynamics of the atmospheric moisture cycle may result. The consequences of this better understanding would include improved rainfall predictions over land, and a more complete meterological model with which to base predictions of global climate change.
In co-pending application number PCT/AU92/00225, entitled Oceanographic and Meterologicai Data, a method was disclosed of determining a with range of oceanographic and meteorological factors. In particular it was disclosed how an Over-the-Horizon Radar (OTHR) could be used to determine such factors as wind direction, wind speed regime, sea state, dominant wave period, significant wave height, dominant wave direction, nondirectional wave spectrum, directional wave spectrum and extended directional wave spectrum.
An OTHR is particularly useful in this regard due to its wide area of coverage, ability to record and analyse measurements in real time and high spatial and temporal resolution. The only competing technology is that of satellite-born sensors, but these cannot achieve real-time measurements.
It is a discovery of the inventor that an OTHR can be used to remotely detect rainfall. This is particularly useful since there currently exists no operational method of remotely detecting the presence of rainfall. Although the presence of rain-bearing clouds can be determined using satellite-borne sensors, and indirect techniques for inferring likely rainfall from cloud temperatures have been developed, the actual occurrence of precipitation can not be established with existing technology.
The actual discovery is that the presence of rainfall in an area interrogated by an OTHR is evident in the nature of the sea clutter Doppler spectrum. The invention discloses a method of extracting this information from the spectrum. In order to comprehend the invention, it is first necessary to understand the scattering of radio waves from the sea surface and the signal processing employed to obtain HF Doppler spectra from the radar echoes. This is achieved in the co-pending application referenced above.
A typical sea clutter spectrum shows a spread of signals ranging between minus 1 or 2 Hertz and plus 1 or 2 Hertz. It has long been known that one of the effects of rain on the sea surface is the damping of wind-generated surface gravity waves. This invention is based on the concept that this wave damping necessarily results in changes to the Doppler spectrum of the reflected radio waves. In particular, significant changes to the "wings" of the Doppler spectrum arise due to the attenuation of the shorter gravity waves by the rain.
Several mechanisms have been proposed to account for the observational evidence that rainfall attenuates surface gravity waves. Manton, M. J. "On the attenuation of sea waves by rain", Geophysical Fluid Dynamics, vol. 5, pp. 249-260, 1973 identified three possibilities:
The entrainment of fluid by drops descending through the water produces vertical mixing, as originally suggested by Reynolds, O., Papers on Mechanical and Physical Subjects, vol. 1, p. 86, Cambridge University Press, 1900. This entrainment will generate sub-surface vortex rings which propagate downward, producing a thin turbulent mixing layer which will interact with the surface gravity wave velocity field and cause attenuation. The microscate processes can be parameterised as an eddy viscosity.
The momentum flux into the water body produces a resultant vertical stress on the water, and a horizontal stress if the mean wind velocity is non-zero. The vertical stress gradient must be balanced by a vertical pressure gradient, which is equivalent to an increase in the gravitational acceleration below the water surface. The surface wave dynamics, as governed by the boundary conditions and embodied in the dispersion relation, are modified accordingly.
If the mean wind velocity is non-zero, the horizontal momentum flux must be reflected in the fluid momentum transport by surface currents and surface gravity waves.
Manton proceeded to argue that only the first of these mechanisms could contribute appreciably in practice. Le Mehaute, B. L. and Khangaonkar, T. "Dynamic interaction of intense rain with water waves", J. Phys. Ocean, vol. 20, pp. 1805-1812, 1990 carded out a detailed analysis of the transfer of horizontal momentum and showed that the wave surface dynamics enhanced momentum transfer, so that wave amplification becomes possible, not just attenuation. Finally, Nysten, J. A. "A note on the attenuation of surface gravity waves by rainfall", J. Geophys. Res., vol. 93, No. C10, pp. 18353-18355, 1990 used energy arguments to compute e-folding times for attenuation.
It is an object of this invention to provide a method whereby this discovery can be practically exploited to provide a method of remotely detecting the presence of rainfall.